The four photoreceptors comprising the median eye of the giant barnacle project to the supraesophageal ganglion where the visual information is processed. We can see and record from photoreceptors, even near their synaptic terminals, and from ganglion cells that we conclude to be second- and third-order neurons in the visual pathway. Simultaneous recordings made from pairs of these three cells under various experimental conditions have enabled us to study their properties and to determine how they are connected. We can largely explain how a depolarizing, sustained photoreceptor signal is converted to a transient, amplified off-response in third-order cells. The essential features of this cell circuit are: 1) the photoreceptor is an inhibitory neuron; 2) second-order neurons are excitatory, decrementally-conducting neurons that rest in the dark at a relatively depolarized membrane potential and release transmitter in the dark in an ongoing fashion; 3) the synapses from the second- to the third-order cell can amplify presynaptic voltage changes. We are studying the properties of the two synapses in this circuit in order to understand better how visual system maintains its sensitivity to small changes of illumination over a wide range of light intensities.